The broad long-term objectives of the application are to determine the mechanisms by which the Severe Acute Respiratory Syndrome (SARS)-associated coronavirus (SCoV) causes acute lung injury and to identify new therapeutic strategies. The central hypothesis of this application is that the pathogenicity of SCoV is caused, at least in part, by the expression of factor(s) that interfere with the host response to viral lung injury. This hypothesis is based on our combined observations that a) expression of surfactant protein-B and -C, which is essential for lung function, is decreased in the lungs of SARS patients, b) SCoV inhibits both virus- and interferon-dependent signaling, and c) a non-structural protein of SCoV recapitulates in isolation the inhibitions observed in SCoV-infected cells, and also inhibits cell proliferation, which is required for alveolar repair. Aim#1: analyze the response of lung alveolar cells to SCoV infection; Aim #2: identify the mechanism by which SCoV impedes the function of lung alveolar cells; and Aim #3: produce and validate attenuated SCoVs. The research design involves the analysis of viral replication, virus- and interferon- dependent signaling, surfactant protein expression and cellular proliferation in SCoV-infected cells. This analysis will be conducted in vivo, using lung tissues from SARS cases and from experimentally infected cynomolgus macaques; and in vitro, using primary human alveolar cells and lung cell lines. The role of the non-structural protein in virulence will be tested by introducing inactivating mutations in the context of recombinant SCoVs by reverse genetics. Viable mutants will be propagated in interferon-deficient cells. Their attenuated character will be determined by comparison to wild-type virus in vitro and in vivo, using cynomolgus macaques as the model organism. Together, the proposed investigations are expected to uncover how SCoV causes acute lung injury and to identify promising therapeutic targets. We expect that our studies on SCoV virulence will serve as a model for other human coronaviruses and respiratory viruses that evade the innate immune response. Understanding this mechanism of SCoV virulence is significant for public health because it will provide knowledge essential for the development of new therapeutic strategies to fight SARS, which has affected thousands of people and has caused major economic disruptions.